Oscillating devices which may include haptic actuators, speakers and crystal oscillators are operated with a frequency that is characteristic of the device. Depending on the conditions, this frequency may drift for example as a function of temperature, external load conditions, or time. For this reason, it is necessary to use a control system capable of tracking the operation frequency of the device.
Haptic actuators are commonly used with electronic devices to provide the user with a sensory signal also referred to as haptic feedback. For example, mobile phones are often equipped with a haptic actuator which vibrates to provide a notification for a user upon the arrival of a text message or other similar event. To this end different types of haptic actuators have been developed, among which are the linear resonant actuator, LRA, and piezo-electric actuators.
LRAs are based on an inductive coil (solenoid) coupled to a spring holding a permanent magnet. In operation, the spring and mass system move along a single axis. When a current is passing in one direction through the coil it creates a magnetic field that repels the magnet. When passing the current in the other direction the magnetic field attracts the magnet. Hence, the transfer of energy between the spring and the coil generates oscillations. The system has a mechanical resonance frequency typically in the range of 50-300 Hz. At the resonant frequency, the push-pull drive voltage produces the maximum linear deviation of the sprung mass.
Following Lenz's law, upon oscillation of the mass, a back electromotive force, BEMF, is generated across the actuator that opposes the voltage of the source that created it. When the mass moves through the coil it follows a simple harmonic motion, which causes it to induce a sinusoidal voltage in the solenoid. This BEMF is strongest at the resonance frequency of the system. The BEMF is proportional in the first order to a drive signal driving the LRA. However, the BEMF amplitude is dependent on the weight and strength of the magnet and the number of turns of the solenoid coil. This means that the BEMF varies across LRAs and across temperature and other electro-mechanical parameters.